That type of material is desirable, in particular for positioning on the inner surfaces of casings in which revolving parts are turning (of the compressor or turbine blade type, for example). Said material can be worn away by the extremities of said revolving parts when interference occurs. Thus, the deleterious effects of said interference are minimized both as regards the structure of said extremities of said revolving parts, and as regards said inner surfaces of said casings.
That type of material advantageously has the following specifications:                clearly, it has to have good abradability;        it must also be resistant to the temperature of the media into which they are incorporated (500° C. to 1200° C., for example, inside the casings of aircraft engines);        it must also resist erosion by repeated strikes from abrasive particles (detached during the interference mentioned above);        its production costs and assembly costs must also be as low as possible;        finally, it is preferably also self-brazing.        
Two types of abradable metallic materials have principally been proposed in the prior art: those generated in situ by projecting (plasma, . . . ) hot metal powders and those with a honeycomb structure which are transferred to and brazed onto the surfaces to be protected.
Those two types of abradable metallic metals are not completely satisfactory, in particular as regards the above specifications.
The first type is associated with a projection technique that is expensive both as regards the projection time and as regards the need for machining following projection, in order to control the thickness of the projected deposit.
The second type has a limited field of application. It is only suitable for wipers for labyrinth seals or for shrouded blades.
French patent application number FR-A-2 223 473 describes porous seals (the porosity in question is an open porosity), consisting of a nickel alloy (nickel is always the major constituent (by weight)) which includes oxides (intentionally formed during the second principal step of the process for producing said alloy). Said nickel alloy can include boron nitride. The alloy fills honeycomb structures.